Local authorities reported 25 deaths on Wednesday, but there are fears more bodies will be found.
Saving lives was the “top priority,” Chinese President Xi Jinping was quoted as saying.
The floodwaters inundated the underground system, where hundreds of people were trapped on trains and also in tunnels, as reported by state media and eyewitnesses on social media. Videos showed water up to people’s shoulders.
During the evacuations, around 600 seriously ill patients also had to be brought to safety from a hospital at Zhengzhou University. The electricity in the hospital was out.
“This kind of rainfall is usually seen once in 100 years. The situation is grim,” said a press release from the flood control headquarters in Zhengzhou, the provincial capital of Henan, according to China Daily.
Many streets in Zhengzhou were flooded. Cars and buses were stuck. Water and electricity supplies were cut off in many places. Traffic was practically paralyzed. The underground network came to a standstill, and high-speed trains were suspended. Zhengzhou airport suspended all incoming flights until 12 pm (0400 GMT) Wednesday, broadcaster CGTN reported.
In the neighbouring city of Gongyi, the authorities reported four fatalities.
The rains in Henan were triggered by typhoon In-Fa, which also caused severe weather in the provinces of Zhejiang and Fujian.
Henan province had already been hit by heavy rains since last week. The water levels of tributaries of the Yellow and Hai rivers exceeded alert levels. According to CGTN the Guojiaju dam near Zhengzhou collapsed at 1:30 am local time.
Further heavy rainfall and flooding was expected. More than 100,000 people have already been brought to safety in evacuations over the past few days. Damage to agriculture runs into the millions.
Severe storms were also reported in southern China, where another typhoon, named Cempaka, made landfall near the city of Yangjiang in Guangdong province with winds of up to 110 kilometres per hour.
In northern China, there was also severe flooding in Inner Mongolia.